In the domain of ultra high frequency and radio frequency (RF) circuitry, it is often desirable to generate one or more attenuated RF signals in secondary couplings from a common RF signal received by a primary coupling element.
For example, an RF coupler is a passive device that may be used to control the amplitude and direction of radio frequency signals in a transmission path between circuit modules. An RF coupler may commonly be configured as a stripline coupler, a microstrip coupler or the like. A stripline coupler comprises generally two parallel strips of metal on a printed circuit board. A stripline coupler ordinarily functions as an RF signal attenuator, that is, a device for generating a controlled amount of signal power transfer from one transmission path to another to provide one or more reduced amplitude RF signals.
In prior art radio frequency couplers, stray capacitances and inductances can result in undesirable cross interference between secondary coupling elements since the secondary elements are generally positioned in parallel on opposite sides of the primary coupling element. This can adversely affect circuit performance. The need for reducing cross interference between coupling elements often results in increasing the distance or amount of isolation between the parallel conducting elements. However, increasing the amount of dielectric isolation between parallel conducting coupling elements can result in increasing the amount of parasitic capacitance, thereby reducing the efficiency of the circuit, and also severely inhibiting desired control of RF signal attenuation.
In prior art RF configurations, the need for increased isolation to reduce the effects of stray capacitance and unwanted electromagnetic coupling between secondary coupling elements not only would adversely affect coupler performance but may impose severe speed limitations on high speed digital circuits which are combined with RF circuitry. For example, it is known that the speed of wave propagation in parallel conductor lines is only about two thirds the speed of light due to the solid dielectric spacing material.
A further drawback of prior art stripline couplers is the tendency of the RF coupling to develop parasitic oscillations due to irregularities in the conductor paths. This problem is due in part to the configuration of prior art RF couplers wherein the coupling elements are generally elongated metallic strips which are disposed in parallel. In most prior art RF couplers, an elongated primary coupling element or transmission line is sandwiched between two elongated secondary coupling elements which are disposed in parallel on opposite sides of the primary coupling and extend along the entire length of the primary coupling. This configuration has severe drawbacks in terms of increasing parasitic oscillations due to the elongated parallel conductive paths. A further drawback of this configuration is an increase in parasitic capacitances.
Accordingly, it is apparent that what is needed is an RF coupling structure which minimizes stray capacitances and parasitic oscillations and which also minimizes cross interference between coupled signals. It is also desirable to control the length and gap of the coupling interface between a primary coupling element and each secondary coupling element to control the electromagnetic coupling effect and thereby provide more precise attenuation of the RF frequency while at the same time minimizing the space required for the coupler. Longer length couplers are also desirable to enhance the coupler's directivity i.e. it is important to minimize the amount of reflected energy from the coupler's output that comes back into the coupler.